1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet having a superior magnetic characteristic and used for an iron core of transformers.
2. Description of the Related Art
Grain-oriented electrical steel sheets are mainly used as an iron core material for transformers and other electrical equipment and must have superior magnetic characteristics, including magnetic exiting and watt-loss characteristics.
The exiting characteristic is usually represented by the value B.sub.8, i.e., a magnetic flux density when a magnetic field of 800A/m is applied, and the watt-loss characteristic is usually represented by the value W.sub.17/50, i.e., a watt-loss value per 1 kg of a magnetic material when magnetized to 1.7 Tesla (T) under a frequency of 50Hz.
The flux density is the strongest factor dominating the watt-loss, and usually, the higher the flux density the better the watt-loss characteristic, although a higher flux density is occasionally accompanied by a coarsening of the secondary-recrystallized grains and resultant degradation of the watt-loss characteristic. The magnetic domain control, however, ensures an improved watt-loss characteristic regardless of the size of the secondary-recrystallized grains.
The magnetic characteristics of a grain-oriented electrical steel sheet are obtained through a Goss-orientation having a {110} plane parallel to the sheet surface and a &lt;001&gt; axis in the rolling direction, which is established by a secondary recrystallization occurring during a final annealing step. To obtain a good magnetic characteristic, the axis &lt;001&gt;, i.e., an axis of easy magnetization, must be precisely aligned in the rolling direction. The orientation of secondary-recrystallized grains is greatly improved by a process in which MnS and AlN, etc., are used as inhibitors and a final cold rolling is carried out at a severe reduction rate. This also leads to a remarkable improvement of the watt-loss characteristic.
In the production of a grain-oriented electrical steel sheet, a hot-rolled steel sheet is usually annealed to obtain a uniform microstructure and effect a precipitation treatment, etc. For example, Japanese Examined Patent Publication (Kokoku) No. 46-23820 discloses a process using AlN as the major inhibitor, in which a treatment for AlN precipitation is effected during an annealing of a hot-rolled sheet, to control the inhibitor.
A grain-oriented electrical steel sheet is usually produced through a process including main process steps such as casting, hot rolling, annealing, cold rolling, decarburization annealing, and final annealing. Such a process consumes a large amount of energy and the production costs are higher than those of a process for producing common steels.
Recent studies of this energy consuming process have concluded that a simplification and omission of process steps are necessary, and to this end, Japanese Examined Patent Publication (Kokoku) No. 59-45730 proposed a process using AlN as the major inhibitor, in which the AlN precipitation is effected during a high temperature coiling after hot rolling as a substitute for a separate AlN precipitation treatment step. This process ensures a certain level of magnetic characteristics without a separate annealing step of hot-rolled sheet, but a 5- to 20-ton hot coil adopted in most cases has locally different heat histories in one coil, which make a nonuniform AlN precipitation unavoidable, with the result that the magnetic characteristic of a final product sheet varies from place to place in a hot coil, and thus the product yield is lowered.
Japanese Examined Patent Publication (Kokoku) No. 54-13846 discloses another process using AlN as an inhibitor, in which a grain-oriented electrical steel sheet having a high magnetic flux density is obtained through single cold rolling step using a severe reduction of from 81 to 95%, and reports that the magnetic characteristic is improved by a rapid cooling after the annealing of a hot-rolled sheet and an aging treatment performed during a cold rolling using such a severe reduction.
Further, Japanese Examined Patent Publication (Kokoku) No. 56-3892 discloses a process for producing a grain-oriented electrical steel sheet using two or more steps of cold rolling, in which a steel sheet is rapidly cooled after an intermediate annealing, prior to a final cold rolling, and subjected to an aging treatment during the final cold rolling to improve the magnetic characteristic, and Japanese Unexamined Patent Publication (Kokai) No. 58-25425 discloses a process for producing a grain-oriented electrical steel sheet using two steps of cold rolling including a final cold rolling carried out at a reduction of from 40 to 80%, in which an aging treatment is performed during the first and the second steps of cold rolling, to improve the magnetic characteristic.
These processes using an aging treatment, however, cannot ensure a stable production of a steel sheet having a superior magnetic characteristic, through a single step of rolling and without an annealing of a hot-rolled sheet.